1. Field of the Invention
The present invention relates generally to satellite communication systems and networks. More particularly, the present invention relates to determining the distances between mobile stations and satellites and the rates of change of those distances.
2. Description of the Related Art
A typical satellite-based communications system comprises at least one terrestrial base station (hereinafter referred to as a gateway), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system.
While the advent of the mobile user terminal has freed users to take their phones wherever they go, it has also imposed upon the satellite communications carrier the burden of determining the location of the user terminal. The need for user terminal position information stems from several considerations. One consideration is that the system should select an appropriate gateway for providing the communications link. One aspect of this consideration is allocation of a communications link to the proper service provider (for example, a telephone company). A service provider is typically assigned a particular geographic territory, and handles all calls with users in that territory. When communications are required with a particular user terminal, the communications system can allocate the call to a service provider based on the territory within which the user terminal is located. In order to determine the appropriate territory, the communications system requires the position of the user terminal. A similar consideration arises when calls must be allocated to service providers based on political boundaries or contractual services.
Several methods of determining the location of the user terminal involve measuring the distance (range) between the user terminal and the satellite and the rate of change of that distance (range rate). When these measurements are combined with other data, such as the position of the satellite, the location of the user terminal can be determined precisely. Techniques for determining user terminal location using satellite-user terminal range and range rate are disclosed in commonly-owned, copending applications entitled "Position Determination Using One Low-Earth Orbit Satellite," Ser. No. 08/723,751; and "Unambiguous Position Determination Using Two Low-Earth Orbit Satellites," Ser. No. 08/723,725.
In the above-described patent applications, the satellite-user terminal range and range rate are ascertained based on measurements of communications signals that are taken at both the user terminal and the gateway. In general, these measurements are not taken simultaneously. Therefore, the satellite moves between the user terminal measurement and the gateway measurement. What is needed is a way to account for the satellite motion in order to obtain more accurate measurements, which will in turn yield more accurate position information for the user terminal.